The present invention relates to a device for detecting thickness of a fabric to be stitched in a sewing machine, the device serving as a means for obtaining information about the fabric thickness and automatically controlling of a thread tension.
There has been conventionally proposed a method for obtaining signals of upper and lower positions of a presser bar lifting lever by means of a potentiometer for automatically giving information about the fabric thickness to a computer circuit of the sewing machine.
A further method has been proposed for inputting numerical stepwise information in response to a thickness of thin, medium and thick fabrics by means of key operations for giving fixed information about the fabric thickness to the computer circuit.
However, when an exchangeable presser foot is used, a standard point of the upper and lower positions of the lifting lever of the presser bar is changed due to differences in plate thicknesses of the metallic presser foot. Therefore, an adjustment should be effected at a zero point for detecting the fabric thickness each time when the presser foot is exchanged.
On the other hand, the above mentioned further method has not satisfied all requirements, because it could not exactly follow changings of the fabric thicknesses during stitching as thickness varies.